Rubber-reinforcing glass fiber treatment agent, rubber-reinforcing cord using the fiber treatment agent, and rubber product

ABSTRACT

There are provided a rubber-reinforcing glass fiber treatment agent capable of improving various properties, in particular the oil resistance, of a rubber product, a rubber-reinforcing cord using the fiber treatment agent, and a rubber product having high oil resistance. The rubber-reinforcing glass fiber treatment agent contains a resorcin-formaldehyde water-soluble condensate and a soap-free acrylonitrile-butadiene copolymer latex. Here, “soap-free” means that polymerization has been carried out using, instead of a conventional emulsifier or surfactant, a polymerization initiator such as potassium persulfate, a styrenesulfonate, an acrylic or allylic reactive emulsifier or water-soluble polymer, or a hydrophilic comonomer such as a water-soluble oligomer or acrylic acid.

TECHNICAL FIELD

The present invention relates to a rubber-reinforcing glass fibertreatment agent, a rubber-reinforcing cord obtained by treating glassfibers using the fiber treatment agent, and a rubber product such as atiming belt or a tire containing the rubber-reinforcing cord.

BACKGROUND ART

Rubber-reinforcing cords comprised of a core comprised of glass fibersor organic fibers made of rayon, nylon, a polyester or the like, and arubber coating on the surface of the core containingresorcin-formaldehyde which has high affinity to the matrix rubber areembedded in rubber products such as timing belts and tires. It is knownthat in the case that such a rubber product is placed under ahigh-temperature high-humidity environment, or is used in oil, therubber coating rapidly deteriorates, resulting in a marked drop in thestrength of the rubber product. Moreover, in the case that such a rubberproduct is used under a low-temperature environment, the matrix rubberand the rubber coating will break upon being subjected to impact due tobeing brittle, and hence there will again be a marked drop in thestrength. For example, timing belts used in a cold region are used undera harsh environment, for example being subjected to impact while stillbrittle upon engine startup, and then subsequently being subjected to ahigh temperature due to waste heat from the engine. In particular, inrecent years there has been a trend toward further increasing thedensity in engine compartments, and hence timing belts are used underyet higher temperature environments.

The rubber coating is formed by applying a solution (hereinafterreferred to as a “fiber treatment agent”) containing an essential rubbercomponent comprised of a resorcin-formaldehyde water-soluble condensate(hereinafter referred to as an “RF condensate”), and if appropriateanother rubber component comprised of an acrylonitrile-butadienecopolymer latex (hereinafter referred to as an “NBR latex”), avinylpyridine-styrene-butadiene copolymer latex or the like, and othercomponents such as age resistors, emulsifiers and/or surfactants ontofibers that form the core, and drying and thus curing. Arubber-reinforcing cord obtained by impregnating such a fiber treatmentagent into glass fibers is described in Japanese Laid-open PatentPublication (Kokai) No. H1-221433.

Glass fibers used as the core have properties such as having a hightensile strength, having a high modulus and hence little temperaturedependence, exhibiting almost elastic deformation upon repeatedstretching, and having good dimensional stability to moisture and heat.These properties are particularly desirable for a rubber-reinforcingcord. On the other hand, one of the serious drawbacks of glass fibers isbeing extremely weak to friction between filaments, resulting in theflexural fatigue resistance, which is an important property required ofa rubber-reinforcing cord, being poor. Moreover, another drawback isthat adhesiveness to rubber is poor. Consequently, in the case of usingglass fibers in a rubber-reinforcing cord, to improve the adhesivenessto the matrix rubber, and to improve the flexural fatigue resistance, itis essential to form a rubber coating.

On the other hand, with a rubber-reinforcing cord in which organicfibers are used as the core, adhesiveness to the matrix rubber can besufficiently secured through the fiber treatment agent only penetratingin two or three layers from the outermost layer of filaments (where a“filament” is the smallest fiber unit). In the case that the fibertreatment agent penetrates in as far as deep layers, the flexuralfatigue resistance may conversely drop, and hence the attachment rate ofthe fiber treatment agent in the rubber-reinforcing cord is oftenadjusted to be not more than 10 wt % in terms of solids.

However, with a rubber-reinforcing cord in which glass fibers are usedas the core, to prevent abrasion between filaments, it is necessary tomake the fiber treatment agent penetrate in as far as the innermostlayer of filaments, and hence the attachment rate of the rubber coating(the attachment rate in terms of solids after drying and curing) isnecessarily increased to 15 to 25 wt %. A rubber-reinforcing cord inwhich glass fibers are used as the core is markedly different to arubber-reinforcing cord in which organic fibers are used as the core inthis respect. The properties of a rubber-reinforcing cord in which glassfibers are used as the core are thus greatly affected by the propertiesof the fiber treatment agent used in treating the core.

With regard to rubber-reinforcing cords in which glass fibers are usedas the core, and a fiber treatment agent containing an NBR latex is usedas the fiber treatment agent, the present inventors carried outassiduous studies with an aim of improving various properties, and as aresult, focusing on the state of the NBR latex in the fiber treatmentagent, discovered that the oil resistance of a rubber product can bemarkedly improved by changing this state. That is, the present inventorscarried out various experiments based on the hypothesis described below,and as a result made new findings for improving various properties, inparticular the oil resistance, of rubber products.

An aqueous solvent is generally used in a fiber treatment agent, thisbeing for reasons such as handling being easy. An NBR latex will notdissolve or disperse in an aqueous solvent, and hence is first made intoa latex by being treated with a low-molecular-weight emulsifier orsurfactant. However, the low-molecular-weight emulsifier or surfactantwill migrate together with the aqueous solvent toward the surface layerof the rubber coating when the rubber coating is formed. If a largeamount of the low-molecular-weight emulsifier or surfactant is presentat the surface of the rubber coating, then the adhesiveness to thematrix rubber, or the adhesiveness to an adhesive applied onto therubber coating will drop. To improve the properties of a rubber product,it is essential to improve the adhesiveness between therubber-reinforcing cords and the matrix rubber. It is thus effective toreduce the amount of emulsifier or surfactant attached to the NBR latex.

The present invention was accomplished based on the above findings. Itis an object of the present invention to provide a rubber-reinforcingglass fiber treatment agent capable of improving the properties, inparticular the oil resistance, of a rubber product, a rubber-reinforcingcord using the fiber treatment agent, and a rubber product having highoil resistance.

DISCLOSURE OF THE INVENTION

To attain the above object, in a first aspect of the present invention,there is provided a rubber-reinforcing glass fiber treatment agentcontaining an RF condensate and a soap-free NBR latex.

In the first aspect of the present invention, preferably, a content ofthe RF condensate is 3 to 35 wt % in terms of solids, and a content ofthe soap-free NBR latex is 35 to 97 wt % in terms of solids, relative tothe weight of total solids in the fiber treatment agent.

In the first aspect of the present invention, preferably, a content ofthe total solids is 15 to 35 wt %.

To attain the above object, in a second aspect of the present invention,there is provided a rubber-reinforcing cord obtained by treating glassfibers using a fiber treatment agent according to the first aspect ofthe present invention.

In the second aspect of the present invention, preferably, an attachmentrate of the fiber treatment agent in terms of total solids is 10 to 30wt %.

To attain the above object, in a third aspect of the present invention,there is provided a rubber product containing a rubber-reinforcing cordaccording to the second aspect of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description will now be given of a preferable embodiment ofthe present invention.

A fiber treatment agent according to the present embodiment contains anRF condensate and an NBR latex as conventionally, but is characterizedin that as the NBR latex a soap-free one is used. Here, “soap-free”indicates that polymerization has been carried out using, instead of aconventional emulsifier or surfactant, a polymerization initiator suchas potassium persulfate, a styrenesulfonate, an acrylic or allylicreactive emulsifier or water-soluble polymer, or a hydrophilic comonomersuch as a water-soluble oligomer or acrylic acid. An example of such asoap-free NBR latex is “Nipol SX1503” made by ZEON Corporation. By usingthe soap-free NBR latex, the adhesiveness between a rubber-reinforcingcord and a matrix rubber can be increased, and as a result variousproperties of a rubber product can be improved, in particular the oilresistance can be markedly improved.

The soap-free NBR latex does not contain an emulsifier or surfactant,and hence will hardly disperse in the fiber treatment agent as it is. Inthe case of using glass fibers as a core, it is essential to make thefiber treatment agent reach as far as the innermost layer of glassfibers, and hence in the present embodiment, it is preferable to blendan acrylic alkali-soluble resin or the like into the fiber treatmentagent in an amount of 0.1 to 10 wt % in terms of solids relative to theweight of solids of the NBR latex. The above-mentioned Nipol SX1503(solid content 42%) already contains such an amount of an acrylicalkali-soluble resin.

The content of the RF condensate in terms of solids relative to theweight of total solids in the fiber treatment agent is preferably 3 to35 wt %. If this content is less than 3 wt %, it will no longer bepossible to attach the RF condensate onto the surface of the glassfibers uniformly, and hence the adhesion between the matrix rubber andthe rubber-reinforcing cord will drop. On the other hand, if thiscontent exceeds 35 wt %, then the rubber coating will become too hard,and hence the flexural fatigue resistance of the rubber-reinforcing cordwill be prone to being insufficient.

Moreover, the content of the soap-free NBR latex in terms of solidsrelative to the weight of total solids in the fiber treatment agent ispreferably 35 to 97 wt %. The NBR latex is the principal component thatdetermines the properties of the rubber-reinforcing cord, and is betterin terms of oil resistance, abrasion resistance and aging resistancethan other rubber components. For these functions to be exhibitedsufficiently, it is preferable for the content of the NBR latex to be atleast 35 wt %. On the other hand, if the content of the NBR latexexceeds 97 wt %, then the content of the RF condensate will become lessthan 3 wt %.

The RF condensate can be obtained by reacting together resorcin andformaldehyde under the presence of an alkaline catalyst such as analkali hydroxide, ammonia or an amine. Moreover, the RF condensate ispreferably a water-soluble initial addition condensation product (resol)between resorcin and formaldehyde that has an abundance of oxymethylgroups, and preferably has a resorcin to formaldehyde molar ratio of1:0.5˜2.5. Moreover, RF condensates are commercially sold as resol typeresins or novolak type resin, and these may also be used. Out of suchcommercially sold ones, a water-soluble type having a solid content of 5to 10%, particularly preferably 8 wt %, is preferable.

Other components such as a latex stabilizer or an age resistor may beblended into the fiber treatment agent as required. By blending in sucha latex stabilizer, age resistor or the like in an amount of 0.1 to 10wt % relative to the weight of total solids in the fiber treatmentagent, the dispersibility of the NBR latex in the fiber treatment agentcan be improved, without inhibiting the polymerization reaction of theNBR latex.

Moreover, the solvent of the fiber treatment agent may be water only asconventionally, but to improve the dispersibility of the soap-free NBRlatex and the RF condensate, it is preferable to blend ammonia intowater as appropriate.

The total solid content of the fiber treatment agent is preferably 15 to35 wt %. The total solid content is proportional to the viscosity of thefiber treatment agent. Consequently, if this content is less than 15 wt%, then the viscosity of the fiber treatment agent will become too low,and hence it will become necessary to carry out application a pluralityof times to sufficiently attach the RF condensate and the soap-free NBRlatex to the glass fibers, and thus the efficiency of production of therubber-reinforcing cord will drop. On the other hand, if the total solidcontent exceeds 35 wt %, then the viscosity of the fiber treatment agentwill become too high, and hence it will become difficult for thesoap-free NBR latex to reach as far as the innermost layer of glassfibers uniformly.

There are no particular limitations on the method of applying the fibertreatment agent onto the glass fibers, but in view of making the fibertreatment agent reach as far as the innermost layer of glass fibers, itis thought that an immersion method in which the glass fibers areimmersed in the fiber treatment agent for a certain time period is best.Excess fiber treatment agent attached to the glass fibers after theglass fibers have been taken out from the fiber treatment agent isremoved as appropriate, and then the glass fibers are heated, thusremoving the solvent and promoting the polymerization reaction of theNBR latex, whereby a rubber coating is formed. Note that a sizing may ormay not have been applied onto the glass fibers during spinning. Aplurality of the glass fibers that have been coated with the rubbercoating are placed together as appropriate, and twisting is carried out,whereby a rubber-reinforcing cord is formed.

The attachment rate of the fiber treatment agent in terms of totalsolids in the rubber-reinforcing cord is preferably 10 to 30 wt %relative to the total weight of the rubber-reinforcing cord having glassfibers as a core. If the attachment rate is less than 10 wt %, then thesoap-free NBR latex may not sufficiently reach as far as the innermostlayer of glass fibers, and hence the flexural fatigue resistance of therubber-reinforcing cord may drop. On the other hand, if the attachmentrate exceeds 30 wt %, then beyond this the rubber coating will merelybecome thicker on the outermost layer of glass fibers, and hence therewill be little improvement in the properties of the rubber-reinforcingcord.

Rubber-reinforcing cords are embedded in an unvulcanized matrix rubberusing a known method, and then heating and vulcanization are carried outunder pressure, thus producing a rubber product.

There are no particular limitations on the matrix rubber used in therubber product; one having good adhesiveness to the RF condensate andthe soap-free NBR latex may be selected and used as appropriate.Preferable examples include chloroprene rubber, hydrogenated nitrilerubber, and chlorosulfonated polyethylene rubber.

The rubber product is better in terms of various properties than arubber product containing rubber-reinforcing cords in which aconventional NBR latex is used, and has excellent oil resistance inparticular. The rubber product can thus be suitably used as a timingbelt for vehicle engines for which high oil resistance is required. Notethat to cope with the increasing density of engine compartments and theaccompanying temperature increase, in recent years a heat-resistantrubber such as chlorosulfonated polyethylene rubber or hydrogenatednitrile rubber has come to be used as the matrix rubber of timing belts.When embedding rubber-reinforcing cords in such a heat-resistant rubber,to improve the adhesiveness between the heat-resistant rubber and therubber-reinforcing cords, the surface of each of the rubber-reinforcingcords may be treated with an adhesive treatment liquid containing ahalogen-containing polymer or an isocyanate compound. A Chemlok (tradename, made by Lord Corporation) is preferable as such an adhesivetreatment liquid.

The present invention will now be described yet more concretely throughan example and comparative examples.

EXAMPLE 1

Alkali-free glass filaments of diameter 9 μm were spun, and severalhundred of these were bound together using a sizing, thus preparing33.7-tex glass fibers. Three of the glass fibers were combined, and theresulting core was immersed in a fiber treatment agent having thecomposition shown in Table 1 below, and then after being pulled out,excess fiber treatment agent was removed. TABLE 1 Content in Terms ofTotal Component Solids Solids (Parts by Weight) (Parts by Weight) (wt %)Soap-Free NBR Latex 90 37.8 89.9 (Nipol SX1503, Solid Content 42 wt %)RF Condensate (Solid 50 4.0 9.5 Content 8 wt %) 25% Ammonia Water 1 0.30.6 Water 25 — — Total 166 42.1 100.0Note: Total solid content of fiber treatment agent is 25.3 wt %

After that, the glass fibers were subjected to heat treatment at 250° C.for 2 minutes, thus completely removing the solvent, and hence forming arubber coating. For the glass fibers coated with the rubber coating, theattachment rate of the fiber treatment agent in terms of total solidswas measured using a known means to be 20 wt %. Next, primary twistingin the Z-direction (S-direction) of 2.1 twists per inch was applied tothe glass fibers. 11 of the twisted glass fibers were then put together,and secondary twisting of 2.1 twists per inch was applied in theS-direction (Z-direction), whereby rubber-reinforcing cords ofspecification number ECG150 3/11 2.1S(Z) were formed. Ahalogen-containing polymer adhesive liquid (obtained by diluting Chemlok233 (trade name, made by Lord Corporation, solid content 23.5 wt %) withxylene) was applied uniformly onto the surface of each of therubber-reinforcing cords, and heating was carried out to remove thesolvent. The attachment rate of the adhesive in terms of solids was 3.5wt % of the rubber-reinforcing cord including the adhesive after dryingand curing.

The rubber-reinforcing cords were embedded using a known means into amatrix rubber having the composition shown in Table 2 below, and atoothed belt of width 19 mm and length 980 mm was formed. TABLE 2 Partsby Weight Hydrogenated Nitrile Rubber (Zetpol 2020) 100 Carbon Black 40Zinc Oxide 5 Stearic Acid 1 Thiokol (TP-95) 5 Sulfur 0.5 TetramethylThiuram Disulfide 1.5 Cyclohexyl Benzothiazyl Sulfenamide 1

The toothed belt was installed in a running test machine equipped with adriving motor at 6,000 rpm and 120° C., and a 504-hour running test wascarried out with part of the belt always immersed in oil. The length andtensile strength of the toothed belt were each measured before and afterthe test, and the elongation change and the strength retention rate werecalculated. The calculation results are shown in Table 5 below. Thescalculation formulae were as follows.Elongation change (%)={(length of belt after running test−length of beltbefore running test)/length of belt before running test}×100Strength retention rate (%)=(strength after running test/strength beforerunning test)×100

COMPARATIVE EXAMPLE 1

Rubber-reinforcing cords and a toothed belt were prepared as in Example1 except that the fiber treatment agent shown in Table 3 below was used,and a running test was carried out. The results are again shown in Table5 below. TABLE 3 Total Content in Component Solids Terms (Parts by(Parts by of Solids Weight) Weight) (wt %) RF Condensate (Solid Content30 2.4 8.4 8 wt %) Vinylpyridine-Butadiene-Styrene 30 12.0 41.9Copolymer Latex (Nipol 2518FS, Solid Content 40 wt %) DicarboxylatedButadiene-Styrene 15 6.0 20.9 Copolymer Latex (Nipol 2570X5, SolidContent 40 wt %) Chlorosulfonated Polyethylene 20 8.0 27.9 Latex(Esprene 200, Solid Content 40 wt %) 25% Ammonia Water 1 0.3 0.9 Water 4— — Total 100 28.7 100.0

COMPARATIVE EXAMPLE 2

Rubber-reinforcing cords and a toothed belt were prepared as in Example1 except that the fiber treatment agent shown in Table 4 below was used,and a running test was carried out. The results are again shown in Table5 below. TABLE 4 Total Content in Component Solids Terms of (Parts by(Parts by Solids Weight) Weight) (wt %) NBR Latex (Nipol 1562, 90 36.989.7 Solid Content 41 wt %) RF Condensate (Solid 50 4.0 9.7 Content 8 wt%) 25% Ammonia Water 1 0.3 0.6 Water 25 — — Total 166 41.2 100.0

TABLE 5 Elongation Strength Retention Change (%) Rate (%) Example 1−0.03 62 Comparative Example 1 −0.17 44 Comparative Example 2 −0.08 55

INDUSTRIAL APPLICABILITY

The present invention, constituted as described above, produces thefollowing effects.

According to the fiber treatment agent of the present invention, thefiber treatment agent contains an RF condensate and a soap-free NBRlatex. As a result, the dispersed solution of the RF condensate and theNBR latex sufficiently reaches as far as the innermost layer of glassfibers, and hence a rubber coating containing the NBR latex can beformed on the surface of the glass fibers with no uneven presence of ahemulsifier or surfactant. Moreover, the rubber coating has higheradhesiveness to the matrix rubber than a rubber coating using aconventional NBR latex.

Moreover, the contents of the respective components of the fibertreatment agent are adjusted such that the content of the RF condensateis 3 to 35 wt % in terms of solids, and the content of the soap-free NBRlatex is 35 to 97 wt % in terms of solids, relative to the weight oftotal solids in the fiber treatment agent. As a result, problems such asthe rubber coating becoming too hard and the adhesiveness to the matrixrubber dropping can be prevented.

Moreover, the total solid content of the fiber treatment agent, thetotal solid content, is adjusted to 15 to 35 wt %. As a result, theviscosity of the fiber treatment agent can be kept in an optimum range,and hence the fiber treatment agent can reliably be made to reach as faras the innermost layer of glass fibers.

Moreover, the rubber-reinforcing cord is manufactured using a fibertreatment agent as described above. As a result, various properties suchas the heat resistance and flexural fatigue resistance can be improved,and in particular the oil resistance can be improved.

Moreover, the rubber product contains such rubber-reinforcing cords. Asa result, a rubber product such as a timing belt for a vehicle engineused under a harsh environment can be produced.

1. A rubber-reinforcing glass fiber treatment agent containing aresorcin-formaldehyde water-soluble condensate and a soap-freeacrylonitrile-butadiene copolymer latex.
 2. A rubber-reinforcing glassfiber treatment agent as claimed in claim 1, wherein a content of theresorcin-formaldehyde water-soluble condensate is 3 to 35 wt % in termsof solids, and a content of the soap-free acrylonitrile-butadienecopolymer latex is 35 to 97 wt % in terms of solids, relative to aweight of total solids in the fiber treatment agent.
 3. Arubber-reinforcing glass fiber treatment agent as claimed in claim 1,wherein a total solid content is 15 to 35 wt %.
 4. A rubber-reinforcingglass fiber treatment agent as claimed in claim 2, wherein a total solidcontent is 15 to 35 wt %.
 5. A rubber-reinforcing cord obtained bytreating glass fibers using a fiber treatment agent as claimed inclaim
 1. 6. A rubber-reinforcing cord as claimed in claim 5, wherein anattachment rate of the fiber treatment agent in terms of total solids is10 to 30 wt %.
 7. A rubber product containing a rubber-reinforcing cordas claimed in claim 5.